A conventional method for mounting a semiconductor element on a substrate for a circuit by means of wireless bonding includes a method in which a conductive bump is formed on an electrode surface of the semiconductor element, and then the bump is bonded to a bonding pad formed on the substrate; and a method in which a conductive bump is formed on a bonding pad of the substrate, and then the bump is bonded to an electrode surface of the semiconductor element. Such methods are disclosed in JP-A-50-10476 and JP-A-50-160774 (The term "JP-A" as used herein means an "unexamined published Japanese patent application").
However, the former method is poor in production efficiency because it is necessary to form the bump on the electrode surface of the semiconductor element in advance, so that the semiconductor element becomes contaminated and damaged. In addition, both methods require greater alignment accuracy in the bonding step of the semiconductor element.
Recently, with the development of electronic equipment which is thinner and miniaturized, there has been a growing demand for thinning the wiring substrate. In order to form such a package, there is adopted a TAB (i.e., tape automated bonding) method in which a film carrier is used.
A variety of inner lead bonding methods in which a semiconductor element is bonded to a finger-like lead. Among these inner lead are known bonding methods, there has been proposed a method in which a bump (i.e., a protruding electrode) is formed on the electrode surface of a semiconductor element, and a lead provided on a film carrier is bonded to the semiconductor element by way of the bump.
However, the method using the bump provided on the electrode surface requires complex processes comprising forming a contact layer made of metals such as titanium or chromium and a barrier layer made of metals such as copper, platinum, or palladium on the electrode, which prevents the bump metal from being. Sputter etching or evaporation, etc., respectively, are used, followed by forming a bump using, e.g., plating with gold. In addition, using this method, it is difficult to eliminate contamination or damage to the semiconductor device.
Also, there has been proposed a method in which a bump is bonded to a lead provided on a film carrier, however, it also requires complex processes for forming the bump like the above method.
Recently a bumpless film carrier having an anisotropic conductive film has been proposed as shown in JP-A-63-4633. Such an anisotropic conductive film is produced by a method in which conductive particles such as carbon black, graphite, nickel, copper, silver, etc. are dispersed in an insulating resin film and the conductive particles are oriented in the direction of the thickness of the film. A complex manufacturing method is necessary to obtain a homogeneous anisotropic conductive film. Also, when the orientation of the conductive particles is insufficient, inner lead bonding is insufficiently conducted so that the reliability for connecting may be reduced.
On the other hand, when the semiconductor device obtained by conducting the inner lead bonding is connected to an outer substrate such as a package substrate or a case using an outer lead bonding method, there are problems similar to those described above.
Namely, when a semiconductor device is mounted on an outer substrate by means of an outer lead bonding method, a linear connecting lead made of conductive metals, such as copper is used. This connecting lead is formed on a film carrier constituting the semiconductor device, and is connected to a metal wiring unit (i.e., a land portion).
However, such a mounting method requires that cutting or bending is conducted with respect to a connecting lead, so that operating efficiency is poor and connection is difficult. Since the connecting lead protrudes from the film carrier, it is poor in mechanical strength as well as operating efficiency. Thermal connecting methods such as a reflow soldering method are adopted for connecting the lead to the land portion. For example, when a semiconductor device is mounted on a transparent electrode made of metal oxides such as a liquid crystal panel, it is necessary to metalize the upper surface of the transparent electrode in advance so that soldering can be carried out.
Recently, there have been proposed methods in which a film carrier is connected to an outer substrate using an anisotropic conductive film or paint as shown in JP-A-63-34936. However, such methods require high alignment accuracy and lead to high costs.